Copper

Copper is required by both plants and animals. Experts say it makes our vegetables taste better. So, our plants need copper; however, our soil life is easily injured by copper in the wrong form.

We include two copper sources on our worksheets. The OMRI organic farming standard is to use copper sulfate (25% Cu) to build soil reserves, when a soil or tissue test determines the soil needs copper. Our second option is an OMRI approved copper chelate, Biomin copper (4% Cu). Industry practice is to use the copper chelate as a foliar, only after a tissue test determines copper is deficient. The copper chelate is considered too expensive for farm scale soil applications; therefore no manufacturer’s data is available for building soil reserves with copper chelate. The primary reason we include the copper chelate is because of the problems associated with copper sulfate.

Whereas the other major and minor minerals deserve care in handling, copper sulfate is a real problem. It can be absorbed directly through skin. When I used it, I mixed it with boron and applied it to the soil (not on the plants) as a spray, separate from the other minerals. That way I could be more careful with it. Copper sulfate does stabilize in the soil and loses its toxic properties. And, as copper is immobile in the soil, once there is sufficient stabilized copper you don’t need to keep re-applying it.

Copper sulfate is a powerful fungicide. Copper chelate will not harm our friends the fungi. In gardens, it is economical enough to build soil copper levels. In fields, it can be applied as a foliar or to the soil every year at low concentrations. It takes less to do more. University papers say it can be used at 1/6 the rate of copper sulfate, when used as a foliar spray. If you are working a large acreage, you will want a tissue test before spending your money on copper fertilizer. Copper is an expensive element to add to fields. Indeed, it is always better to get a tissue test before applying copper.

There are two major types of copper chelate on the market, one using EDTA, and one using citric acid. Only the citric acid copper chelate qualifies for OMRI certification.

Chelation means that the copper is surrounded by a ring of other atoms or molecules. This ring makes the chelated copper non-reactive compared to CuSO4, and without fungicidal properties. The mechanism by which the plant can then access the copper is complex. There is strong chelation and weak chelation. Basically the citric acid is used as a sort of binder, and the outer ring is an amino acid which the plant wants to absorb. So, the citric acid chelated copper is readily available to plants.

Application rates for copper chelate can vary according to your circumstances. If you have a 4% copper chelate solution, just 3-4 tsp per 1000 sq ft every 4 to 6 weeks applied to plants provides sufficient copper. If you prefer to apply it to soil, use 4-6 tsp per 1000 sq ft. If you want to establish a soil reserve, you can bring the copper level up slowly up to 5 ppm (10 lbs/ac, 6” deep) by applying 2 lbs of elemental copper chelate at a time.

In Australia, Biomin copper is available at a different concentration, not 4%.

Copper requirements are debatable. In a recent Acres article, Hugh Lovel summed up the situation nicely. “Though 2 ppm copper is generally considered adequate, 5 ppm gives more margin and 10 is not harmful unless the soil is extremely light with poor humus reserves.” These are similar to the numbers Neil Kinsey gives us “Anything below 2 ppm copper means deficiency. Five ppm is excellent, but 10+ ppm is still not considered excessive.”

Here are some estimates of copper requirements. These numbers are all based on Mehlich 3 extractions, and are expressed in lbs/ac (6” deep) of elemental copper, which are double the ppm numbers.
In our worksheets we are now targeting 6 to 10 lbs per acre of elemental copper per acre, with a 4 lb/ac application limit. It is wise to limit the per-application amounts of copper sulfate, so as to avoid setting back our soil life too hard. We set an application limit of 4.0 lbs per acre copper per year (16 lbs/ac copper sulfate). If you are using copper chelate, you can use less, so the application limit is 2 lbs/ac.

The values expressed as ranges are in a sense more realistic. In our own experience Mehlich 3 copper tests are not very repeatable. We have seen them vary by up to 40%. Agricultural Laboratory Proficiency (ALP) Program data confirms EDTA intra-lab copper test results vary by +- 20%. It is important to keep this in mind when building copper levels in your soil. Don’t over-react to the first test. Use sensible application limits.

Copper is one of the most immobile elements, and it is quite persistent. On our own land, this year’s soil test showed a less-than-optimum level of copper. It was right on target last year. Where did it go? I don’t think it went anywhere. The soil is producing like never before, the taste has exceeded what I would have thought possible, and the soil test results are showing good levels of about all the nutrients. The soil has settled into a great place. At this point I am thinking that we have reached a nice balance, and that if the soil test results don’t match the targets, maybe something is wrong with the targets or the soil tests. At first, one learns soil analysis by following directions. Later, it becomes just another of many means to understand what the earth and the plants are telling us.

It is a very good idea to keep records of how much copper has been applied. Even if soil tests don’t always show it, previous applications are almost certainly still around. According to two university bulletins, one should stop adding copper for any reason once 30 lbs/ac have been applied, so as to avoid copper toxicity. The Danish environmental standards mandate no more copper applications after 80 lbs/acre (US standards are higher). Sheep are particularly sensitive to copper. If you are grazing sheep, your copper targets will be much lower than anyone else’s. If you are working in an orchard where copper might be applied as a fungicide, you will not need to apply any additional copper.

Not all crops require the same amounts of copper. These crops have especially high copper requirements: alfalfa, barley, blueberry, beet, broccoli, carrot, cabbage, celery, eggplant, flax, lettuce, oats, onion, parsnip, pepper, rye, spinach, sudangrass, tomato, watermelon, and wheat. And, soils respond to copper differently. Soils with lots of organic matter are likely to lock up soil applied copper, so with these soils it is better to apply copper as a foliar spray. Animals and plants have similar copper requirements. When manure is applied, it will likely have enough copper in it to avoid plant deficiency. However, fertilizing for optimum health is much different from fertilizing to avoid deficiencies. In the case of copper, though, these two boundaries are much closer than any other nutrient.